Flowmeters can be used in a plurality of industrial fields such as chemical, petrochemical, oil, gas, energy, paper, water, sewage, food and beverage, and pharmaceutical industries. Flowmeters can be used for determining the flow rate of fluids, e.g., in pipes.
High measurement accuracy is a desired property of flowmeters. A particular type of flowmeter measures the flow rate, such as the mass flow rate, with the help of two temperature sensors. In such a flowmeter, the first temperature sensor can measure the fluid temperature T0 and the second temperature sensor can be heated to a higher temperature T2. The second temperature sensor can be cooled by forced convection which depends on the velocity of the fluid passing the sensor. The heating power needed to maintain the higher temperature T2 can reveal a measure of the flow rate.
However, the measurement and hence the accuracy with which the flow rate is determined may be negatively influenced by mechanisms that influence the heat transfer, e.g. by forced convection, of the heated second temperature sensor to the fluid in an uncontrolled manner. One such mechanism can include a changing of the heat transfer due to a coating on the heated sensor. A coating may, for example, build up during operation in dusty, humid, oily, or sooty environments. Coatings can have a lower heat conductivity as compared to the materials of which the sensors are made. Hence, a reduced heating power may be needed to maintain the higher temperature for a coated sensor. The measured flow rate may then be wrong, for example too low, as compared to the actual flow rate.
It is thus desirable to detect sources of failure of the sensors. It has been proposed to switch the roles of the heated sensor and the unheated sensor for diagnosis, as the coating may be different for sensors operated at different temperatures. However, such a switching can be slow, especially because one sensor would need to cool down to the temperature T0 of the fluid. During the cooling that occurs during the switching, it may be impossible to measure the flow rate, creating what is known as a dead time for the flowmeter. Further, the diagnosis may be unreliable if the flow conditions have changed during the dead time.
Improved methods for diagnosis of sensors in flowmeters and for flowmeters configured for such diagnosis of at least one of its sensors are desired. For example, methods and flowmeters are desired where dead times are avoided and reliable diagnosis is conducted at low costs.